Patent Document 1 (Japanese Unexamined Patent Publication (Kokai) No. Hei 8-79865) describes a structure in which: a diaphragm is provided in a receiver casing; a receiver is attached to the back side of a front casing in such a condition that the opening of the receiver casing faces with two or more sound holes formed on the front casing; and in order to prevent water drops from entering into the inside of the receiver through the holes, a waterproof film having excellent sound transmission performance is provided between the front casing and the receiver (see, e.g., FIG. 1).
Patent Document 2 (Japanese Unexamined Patent Publication (Kokai) No. 2006-262262) describes a structure in which: the front side of a rear casing is covered with a front casing; a circuit board is put in a concave portion of the rear casing; the front side of the circuit board is covered with a waterproof-type key sheet material; the outer edge of the waterproof-type key sheet material is sandwiched between the outer edge of the rear casing and the outer edge of the front casing; an mounting hole is formed in the waterproof-type key sheet material; and a microphone holder is inserted into the mounting hole. In addition, a microphone is mounted in the microphone holder; a waterproof film is attached to the end face of the microphone holder; and a small opening of the microphone holder is closed with the waterproof film (see, e.g., FIGS. 2 to 4).